NAD (new to me): 1980 Marshall JMP 2104 50W (2204 combo)

[2dor]

Finally got the thing recap-ed today.

The Daly ones measured good (tech had them tested again) but I feel safer with the new F&T ones in there now.

Just need to take some time later in the day to retube & bias.

Can't wait to crank this one.

 

[2dor]

So the recap job alone took the B+ reading from 375-380 VDC to 408-410 VDC with the same tubes in the amp.. hot damn..

 

[MirrorProfiles]

So the recap job alone took the B+ reading from 375-380 VDC to 408-410 VDC with the same tubes in the amp.. hot damn..

Do you know what voltages you have going into the phase inverter?

 

[2dor]

Do you know what voltages you have going into the phase inverter?

Haven't measured it yet since I had to put everything back together again for the day but I might since I plan to roll a few tubes through the amp.

 

[2dor]

@MirrorProfiles you planted that seed in my head lol

I fitted my EHX EL34 pair back in (had a JJ E34L) the amp and rebiased the amp.

218 ~ 219 V measured on Pin1 of V3; B+ reading with the EHX EL34 in is about 391 VDC with the bias current at 40 mA. The earlier B+ reading was with the bias pretty low (27 mA), before I got to fiddling around with it to get it to a higher dissipation.

 

[EOengineer]

Recap was a solid move, especially with the F&Ts.

 

[2dor]

Recap was a solid move, especially with the F&Ts.

In this case, it was definitely a solid move.

 

[newholland]

even though im super psyched for you to rock this thing as a head.. i LOVE that combo so much.

 

[Humbug]

So the recap job alone took the B+ reading from 375-380 VDC to 408-410 VDC with the same tubes in the amp.. hot damn..

When in doubt... ;-)

 

[Jack Luminous]

So the recap job alone took the B+ reading from 375-380 VDC to 408-410 VDC with the same tubes in the amp.. hot damn..

So replacing the psu caps changed the B+ voltage despite the old caps measured good ? Seems weird.

 

[2dor]

So replacing the psu caps changed the B+ voltage despite the old caps measured good ? Seems weird.

As weird as it would seem, yes - that's the outcome for my amp.

 

[Jack Luminous]

As weird as it would seem, yes - that's the outcome for my amp.

I think a measurement error, a change in bias or a wall voltage fluctuation is a more likely explanation, applying Occam's razor from a purely technical point of view.

 

[2dor]

I think a measurement error, a change in bias or a wall voltage fluctuation is a more likely explanation, applying Occam's razor from a purely technical point of view.

The bias trim pot was exactly in the same position, same tubes, measurement was consistent; can't say about wall voltage but it's a new house, good wiring & urban area.

 

[Jack Luminous]

Then the cause is not clear but caps are not diodes, there is no voltage drop across caps.

 

[Humbug]

As weird as it would seem, yes - that's the outcome for my amp.

Caps pulling the B+ down is not weird at all. It's pretty high on the list of things I check if an older amp has lower than expected supply voltage.

 

[Jack Luminous]

Caps pulling the B+ down is not weird at all. It's pretty high on the list of things I check if an older amp has lower than expected supply voltage.

Only if the caps don't measure OK and are leaking current, which doesn't seems to be the case here since they were checked OK by the tech (twice apparently). Anyway.

 

[2dor]

Yeah I'm no expert on these can caps but they would have probably caused some trouble sooner rather than later by the looks of it.

In any case, I kept them in a bag just in case I need to move the amp (unlikely) & the new owner would want them for the vintage vibes.

As-is, the recap was a good decision in this case & the B+ increase a much welcomed improvement.

Need to roll some preamp tubes now...